Method of providing refractory metals with protective coatings and resulting article



United States Patent Office 3,352,842 Patented Jan. 9, 1968 METHOD OF PROVIDiNG REFRACTORY METALS WITH PROTECTIVE COATINGS AND RESULT- ING ARTICLE Bruce E. Kramer, Loveland, Ohio, assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Filed Oct. 31, 1963, Ser. No. 320,586

14 Claims. (Cl. 11769) ABSTRACT OF THE DISCLOSURE A multilayer oxidation resistant coating which prevents oxygen ion and substrate metal ion diffusion and the process for its production.

This invention relates to the preparation of oxidation resistant coatings and more particularly to oxidation resistant coatings which retard diffusion.

Throughout history users of metal products have been faced with the problems of oxidation and corrosion. A variety of metal platings and coatings have been developed both to enhance the beauty of metal articles as well as to reduce the susceptibility of the articles to oxidation. Extensive research has been devoted to finding solutions for the problems of oxidation and corrosion of metals, particularly in recent years in view of the high-temperature, corrosive environments encountered in the jet and rocket age. Much of the current research in the field is directed toward the development of coating materials which are in themselves resistant to oxidation. Relatively few of the investigators, however, have been concerned with the problem of ion diffusion through the coatings. Of the few investigations in which diffusion was recognized as the major oxidation problem the research approaches led to attempts to solve secondary problems, such as lattice hole plugging and correction of undesirable oxide-to-metal volume ratios, rather than to the ultimate prevention of oxidation by the elimination of diffusion.

Accordingly, objects and features of this invention include the provision of an improved oxidation and corrosion resistant coating on oxidizable base materials.

Another object of the invention is to provide a coating which resists ion diffusion.

A further object of this invention is to provide diffusion resistant coatings which prevent oxidation of base materials.

Another object of this invention is to provide a coating for a readily oxidizable article which effectively retards oxidation at elevated temperatures.

A further object of this invention is to provide a protective coating for an oxidizable metal article which is resistant to ion diffusion and the oxidation and/r base metal shrinkage resulting therefrom.

Another object of this invention is to provide a diffusion resistant oxidation retarding coating for metal articles susceptible to oxidation and corrosion which is both economical and adapted for use 0n objects of numerous sizes and shapes.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description and appended claims.

This invention proposes the prevention of oxidation of coated articles by means of the elimination of both substrate metal ion and oxygen ion diffusion through the coating. Many of the refractory metals having desirably high strengths at elevated temperatures are vulnerable to oxidation when exposed to an oxygen-bearing atmos phere. It is a well known practice to coat these metals with materials having a high resistance to oxidation, such as chromia, alumina and ceramics. Generally, the coating materials are selected in view of the resistance to oxidation of the coating material itself. In practicing the prior art systems the problem of ion diffusion through the coating material is completely overlooked or neglected. It has been found that some materials are metal deficient semiconductors, P conductors, and when used as coatings on metals allow ions of the substrate to diffuse through the coating and oxidize at the coating-air interface. Other materials are metal excess or vacant anion site semiconductors, N conductors, and when used as a metal coating permit oxygen ions to diffuse inwardly through the coating thereby promoting oxidation at the metalcoating interface. If, then, an N conductor is applied as a coating adjacent to the metal and a P conductor is applied as a second layer over the N conductor, metal and oxygen ions cannot easily diffuse through the coating. Thus, the adverse reaction of oxygen on the substrate metal is eliminated. The specific semiconductors for a given coating must be selected with care to guard against reaction between the substrate and the coating as well as between the two coating layers, Generally, very high melting point materials must be selected for coating because the Tammann temperature, 0.52 Tm where Tm is the melting point in degrees Kelvin, is the temperature of onset of mobility of lattice atoms or ions. This bulk phase diffusion takes place at temperatures greater than 0.52 Tm.

The methods of applying thin layers of metals or compounds to substrates are well known. These include cementing, slurry coating, and heat treating, electroplating, electrophoretic deposition, vapor deposition by both pyrolysis and evaporation and condensation, and flame spraying. Generally speaking, the types of coatings contemplated are thin, 0.0001 to 0.001 inch, and nearly perfect; that is, free from large cracks, pin holes and voids.

A number of examples are set forth hereinafter to further describe this invention and explain the use of its concept, thereby illustrating its uniqueness and utility.

EXAMPLE I Columbium-zirconium alloys when oxidized are known to form a coating of the approximate composition ZrO -6 Cb O This oxide composition is not an effective barrier to diffusion and oxidation and, moreover, it has a relatively low melting point. To provide oxidation resistance for the columbium-zirconium alloy, zirconium is diffused into the surface to form a zirconium-rich zirconiumcolumbium alloy at the surface. The alloy is then oxidized at a temperature above that contemplated for use in a pure, dry oxygen environment for a short time to form a thin adherent oxide film of the approximate composition 6 ZrO -Cb O An alternative method of forming the oxide coating is by anodizing. The semiconductor properties of 6 ZrO -Cb O are then measured. If the coating is found to be a P conductor, its composition must be adjusted, or an addition must be made, to change it to an N conductor. As the second layer of the coating, a P conductor such as Cr O is applied. Oxidation re sistance of this coated columbium specimen is excellent.

One of the materials which forms a solid solution with columbium is hafnium. As an initial coating, hafnium may be diffused into columbium and oxidized to form N semiconductor HfO To complete the diffusion retarding coating system, the hafnium coated article is provided with a coating of a P conductor, such as Cr O as set forth above.

3 EXAMPLE u Titanium and chromium form solid solutions with molybdenum. Since Cr O is a P conductor and TiO is an N conductor, titanium is selected for the first layer. The titanium is difiused into the surface of molybdenum to form a thin titanium coating on the specimen and then oxidized to form TiO A P conductor metal oxide such as Cr O is then applied as the second layer to provide the diffusion-retardant, oxidation-resistant coating.

The materials aluminum, zirconium, hafnium thorium, barium, and beryllium form intermetallics with molybdenum. Any one of these materials may be applied to molybdenum as a thin coating and heat-treated at a temperature sufiiciently low to prevent formation of the respective intermetallic. The coating is then oxidized anodically or by high temperature oxidation to form the respective oxide which is expected to be an N conductor. If it is not an N conductor, the composition thereof may be altered. A P conductor metal oxide is then applied as a second layer.

EXAMPLE III Carburizing a tantalum article forms a TaC coating. The composition of the coating may be adjusted if necessary to assure N conductor properties. As the second layer, a P conductor metal oxide is applied.

EXAMPLE IV Tungsten may be silicided, nitrided, aluminized, or reacted with other materials to form high melting point tungsten compounds which are N conductors. As a surface coating therefor, a P conductor metal oxide is then applied as the second layer of the coating.

EXAMPLE V A good molybdenum coating system currently in use consists of electroplated chromium and an overlay of flame-sprayed aluminum oxide. The two major disadvantages of this coating are that in use at elevated temperatures the chromium deteriorates as a result of diffusion with molybdenum and the alumina spalls as a result of oxidation at the Cr-Al O interface. When the coating is examined in the light of the new concept presented herein, it is apparent that the coating is reversed; that is, alumina is an N conductor which allows oxygen ions to pass through it and chromia is a P conductor which permits metal ions to pass therethrough. An improved molybdenum coating, therefore, is a coating of alumina applied directly to the molybdenum, then chromia or oxidized chromium is applied on the alumina.

EXAMPLE VI A good oxidation resistant coating system for columbium was developed which provided protection up to about 2500 F. This system consists of flame sprayed alumina impregnated with a glass sealant. In view of the new concept herein presented and to make this coating useful to still higher temperatures, the glass sealant is eliminated and a high melting point P conductor is applied over the aluminum.

The examples cited above describe several types of solutions for the problems of inhibiting diffusion-oxidation reactions. It is to be expected that if one approach efiectively protects columbium, it may also work well on tantalum. In the same manner, those coatings which eliminate molybdenum oxidation should he eitective oxidation retardants for tungsten, and vice versa. The instant concept may also be applied to super-alloys. Super-alloys containing chromium, for instance, oxidize to form substantially Cr O at the surface. Since Cr O is a P conductor, chromium continues to difiuse through the oxide coating thereby depleting the substrate of chromium. Such a chromium-depleted substrate has a tendency to crack. In the light of the present invention, it is apparent that chromizing will only delay the process. A favorable solution is aluminizing the article, and thereafter oxidizing the article to form an A1 0 coating, an N conductor. A P conductor coating such as Cr O is then applied over the alumina as a second layer. Such a coating will stop oxidation, not merely delay it.

The singificance of this invention is a new concept in the treatment of oxidation resistance if the invention is racticed in the manner described, and 0.52 Tm is not exceeded during use. A properly coated metal will resist oxidation for an infinitely long period. For shorter time periods 0.52 Tm may be exceeded, thus permitting an increase in the working temperature to which the protected article may be subjected. It is thus apparent that the objects of this invention have been accomplished in the provision of an economical diffusion-resistant, oxidation-retarding coating for metal articles, particularly for use in environments of elevated temperature and corrosive influences.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It should therefore be understood that it is intended that all matter contained in the above description will be interpreted as illustrative and not in a limiting sense.

What is claimed is. 1. An article of manufacture composed of an oxidizable metal substrate selected from the group consisting essentially of Cb, Mo, Ta, and W;

an inner coating, over said metal substrate, of a material selected from the group consisting essentially of HfO TiO A1 0 ZrO BaO, BeO, TaC, material resulting from siliciding, nitriding or aluminizing tungsten, T and 6ZrO -Cb O an outer layer coating of P conductor Cr O whereby oxidation of said substrate is retarded by the prevention of any association of ditfused substrate metal and oxygen ions.

2. A combination of structure of the character of claim 1 wherein said metal substrate comprises colum'bium, and

said inner layer comprises 6ZrO 'Cb O 3. An article according to claim 1 wherein said substrate comprises columbium said inner layer comprises HfO 4. The article according to claim 1 wherein said substrate comprises molybdenum, and

said inner layer comprises TiO 5. An article of the character of claim 1 wherein said substrate comprises tantalum and said inner layer comprises TaC.

6. A combination of structure according to claim 1 wherein said substrate comprises tungsten and said inner layer is a compound chosen from the group of materials resulting from siliciding, nitriding and aluminizing tungsten.

7. An article of manufacture having a substrate comprised of molybdenum rendered substantially free from oxidation at elevated temperatures by virtue of an inner coating of an N conductor material selected from the group consisting essentially of HfO TiO A1 0 ZrO BaO, BeO, TaC, material resulting from siliciding, nitriding or aluminizing tungsten, ThO and 6ZrO -Cb O and an outer layer coating of P conductor Cr O said layers being bonded to said substrate and resistant to diiiusion of molybdenum and oxygen ions therethrough thereby preventing oxidation of said molybdenum at the substratecoating and coating-atmosphere interfaces.

8. An article according to claim 7 wherein said inner layer comprises aluminum oxide.

9. An article of the character of claim 7 wherein said inner layer comprises zirconium oxide.

10. An article of the type set forth in claim 7 wherein said inner layer comprises hafnium oxide.

11. An article according to claim 7 wherein said inner layer comprises thorium oxide.

12. The combination of structure of the character of claim 7 wherein said inner layer comprises beryllium oxide.

13. A process for increasing the oxidation resistance of a metal article selected from the group consisting essentially of Cb, Mo, Ta and W by producing in situ a diffusion barrier on the surface thereof which consists of applying to said metal article a coating of a material selected from the group consisting essentially of aluminum, zirconium, hafnium, thorium, barium and beryllium, oxidizing said coating and thereafter applying a layer coating of P conductor CI2O3 whereby said multilayer barrier inhibits both tendencies toward oxidation and shrinkage of said article by eliminating oxygen ion and substrate metal ion diffusion therethrough.

14. A process of the type set forth in claim 13 wherein said metal article comprises molybdenum and the coating material is chosen from the group of materials References Cited UNITED STATES PATENTS Garner 117-71 X Schulze 29195 X Ishikawa et al. 317-238 X Cochran 11769 Day et a1. 117-69 Linton 117-69 X Heinze 14833.4 McGuire 14820.3 X

20 ALFRED L. LEAVITT, Primary Examiner. I. R. BATTEN, JR, Assistant Examiner. 

